1. Field of the Invention
The present invention relates to a semiconductor device having a capacitor, a bipolar transistor and complementary metal--oxide--semiconductor field effect transistors (MOSFET) and a manufacturing method thereof.
2. Description of the Prior Art
Recently, there has been a demand to add a still larger capacitance to a Bi--CMOSLSI in which complementary metal-oxide-semiconductors (CMOS) marked by large scale integration and low power consumption, as well as a bipolar transistor marked by rapid performance, are formed upon a single semiconductor substrate.
In Japanese Patent Application Laid-open No. 22054/1989, there is disclosed a manufacturing method of a capacitor in a semiconductor device having a MOSFET and a bipolar transistor on a single substrate. In this method, one electrode of the capacitor is formed simultaneously with a gate electrode of the MOSFET, and, after insulating films of the capacitor are formed, an opposite electrode of the capacitor is formed simultaneously with an emitter electrode of the bipolar transistor. The device herein has a structure in which capacitor insulating films are sandwiched by the overlying and underlying electrodes of the capacitor.
However, the capacitor formed in this method occupies a large area, which restricts a large scale integration of the LSI.
Further, in Japanese Patent Application Laid-open No. 291262/ 1994, there is disclosed a method of adding a capacitor with high precision to a Bi--CMOS, wherein a lower electrode of the capacitor is formed over an insulating film and concurrently serves as polysilicon to form gate electrodes of MOSs, while an opposite electrode of the capacitor serves as polysilicon to form a base or an emitter. Nevertheless, this method also has a problem that the capacitor occupies a rather large area.
Meanwhile, as a method of providing a large capacitance without requiring a large area for that, there is, for example, disclosed in Japanese Patent Application Laid - open No. 150955/1988. In this method, as shown in FIG. 19 (a), a polysilicon layer 53 is placed as an intermediate layer. Sandwiching a silicon dioxide film 51, this polysilicon layer 53 and a diffusion region 52 which is set in an epitaxial layer 57 lying over a silicon substrate 56 form a first capacitor. The polysilicon layer 53 and an Al film 54, sandwiching a silicon dioxide film 58, form a second capacitor. The diffusion region 52 and the Al film 54 are connected with each other by means of a contact made through the silicon dioxide film. Accordingly, as shown in FIG. 19 (b ), two capacitors are connected in parallel between an Al film 55 brought from the polysilicon layer 53 and the Al film 54 so that a larger capacitance value can be attained.
However, the silicon dioxide film 58 is a part called as an interlayer insulating film and its film thickness is substantial, which causes a problem that a sufficiently large capacitance cannot be necessarily obtained. Moreover, a resistance of the polysilicon layer section becomes relatively large, resulting in another problem of poor high-frequency characteristics thereof.
Further, another capacitor structure formed on a semiconductor substrate, as shown in FIG. 20 (a), is disclosed in Japanese Patent Application Laid-open No. 75021/1993. In this structure, upon a p --type semiconductor substrate 100, an n-type epitaxial layer 102 is layered, and, over that, an n.sup.+ -diffusion layer 104 is formed. A dielectric layer 106 of silicon dioxide is then formed and a conductive layer 108 of polysilicon is layered in a region where a capacitor is to be formed. Next, a dielectric layer 110 of polysilicon is formed and, after applying a phosphorus glass (PSG) layer 112, a conductive layer 114 of aluminium is layered to connect with an electrode A. The conductive layer 114 of aluminium also connects with an n.sup.+ -diffusion layer through a contact hole and consequently to an electrode B. Accordingly, a capacitor C1 with the dielectric layer 106 and a capacitor C2 with the dielectric layer 110 are connected in parallel so that a capacitance thereof becomes C1+C2, as shown in FIG. 20 (b).
However, the application of this structure to a Bi-CMOS was not described at all in this prior art. Therefore an efficient manufacturing method for a practical use has been kept unknown. Further, this structure gives rise to a problem of poor high-frequency characteristics since a resistance value of the polysilicon section becomes large.